1. Field of the Invention
The present invention relates to an image enhancement device and an image enhancement method of a thermal printer, which can obtain a corrected image with high quality even if quantity of thermal storage to acquire recording density necessary for target gradation data is excessive or insufficient due to the thermal history.
2. Background Art
A conventional thermal history correction method of the thermal printer is carried out by discriminating quantity of thermal storage with referring to the temperature of the thermal head detected by a thermistor attached to the thermal head and the number of printing lines and by controlling power distribution quantity to the thermal head (for example, refer to JP 04-189552. pp 2–4, FIG. 1).
Further, another thermal history correction method is proposed, in which the quantity of heat stored in the thermal head is estimated in a value converted to gradation data, and this estimated value is subtracted from printing gradation data for future printing data (for example, refer to JP 2000-71506. pp 2–7, FIG. 1).
The conventional thermal history correction method as disclosed in JP 04-189552 causes irregular printing, since a difference may occur between a temperature of the thermistor and a temperature of the heat stored in the thermal head as the measuring point of the thermistor becomes far from the thermal head, and such difference makes the temperature correction improper. Further, there is a problem that this method costs much because the method needs a temperature detecting means such as the thermistor.
Further, as for the thermal printer, the recording density is low directly after starting printing, and the recording density becomes high as the thermal storage becomes large. In general, the recording density necessary to acquire target gradation data means the recording density at the time when the heat is stored in some degree. That is, it is difficult to obtain target recording density when the thermal storage is small, which causes irregular printing. The thermal history correction method as disclosed in JP 2000-71506 does not need the temperature detecting means, which enables a low-cost implementation. However, this method merely subtracts estimated quantity of thermal storage which is converted to the gradation from original printing gradation data, so that there is a problem that the method can carry out the correction when the heat quantity exceeds the original gradation data but cannot when the quantity of thermal storage is insufficient to obtain the recording density of the original gradation data. Further, there is another problem that the precision for thermal history correction is not high, since the thermal effect of neighboring heater elements in the main scanning direction is not considered for computing the quantity of thermal storage, though the thermal effect in the sub scanning direction is considered.
The present invention is provided to solve the above problems and aims to obtain a corrected image with high quality even if the quantity of thermal storage to acquire recording density necessary for target gradation data is either excessive or insufficient due to the thermal history.
Further, another object of the present invention aims to obtain the corrected image with higher quality by considering the thermal effect not only in the sub scanning direction but also in the main scanning direction.
Yet further, the present invention is to provide a computing method that enables to reduce the processing time for computing correction data.